The Tunnel Fighter is fighter that excels at extremely close quartered fighting as well as defending themselves in tight spots. 

Level 3: Back to the wall

While Wielding a shield, gain +2 AC if against a wall (not diagonally)

Level 3: Tight Space

At the end of each of your turns, you can choose two tiles that are both within 5 feet of you, these tiles count as space taken by you. A creature may not move through these tiles unless they are 2 or more sizes smaller than you or they are 2 or more sizes bigger than you. These tiles count as space taken by you for purposes of blocking movement for enemies, however allies may freely move through them. They do not count as space occupied for areas of effect or attacks, the space you actually stand in must be within range of the attack or area of effect. Opportunity attacks will not occur if a creature runs past only the additional tiles selected, they must run past your characters real tile.

Level 7: Shield Push

While wielding a shield, you may shove or grapple as a bonus action. A creature has disadvantage on there save against being shoved away but not prone, and you can switch places with a grappled creature for 0 movement.

Level 10: Tighter space

You may now pick 3 tiles for the tight space feature. Additionally, those tiles do not all need to be within 5 feet of you as long as at least 2 are and the other 1 is connected to one of the other two. Lastly, you may now do opportunity attacks from those tiles, even if your weapon would not normally reach.

Level 15: Claustraphobia

If a hostile creature hits you but before they roll damage, you may have the creature re-roll the attack, if they miss the second time, they receive the frightened condition from you until the end of their current turn. You may use this ability only once on any given creature. You may use this ability a number of times equal to your constitution modifier per long rest. 

Level 18: Tunnel Dragging

While wielding a shield, automatically succeed on grapples and shoves.